Cellulose ester resins are becoming the mainstay for polarizing plate protective films. This is mainly because cellulose ester resin is excellent in terms of optical and physical properties advantageous for polarizing plate protective film. However, the common method for manufacturing the film is generally a film casting method which uses a halogenated organic solvent. Recently, halogenated solvent is being restricted in view of environmental considerations and not personal safety. Also, in addition to use in manufacturing processes, regardless of concerted efforts to remove all the halogenated organic solvent in subsequent steps, there is always some residue. This residual solvent is problematic in that it reduces the durability of the manufactured product.
Meanwhile, a melt casting method has been proposed as a method for manufacturing cellulose ester film that does not use a halogenated organic solvent (please refer to Patent Document 1 for an example). The melt casting method is one method of molding a film by extruding the cellulose ester film while performing thermal melting. Because cellulose ester resin is, generally speaking, a material that tends to deteriorate due to heat or mechanical stress, deterioration of the cellulose ester occurs during this thermal melting process, and it is thus difficult to obtain a film that exhibits sufficient mechanical strength. The device used for the thermal melting extrusion process is generally a uniaxial extruder or a biaxial extruder. A biaxial extruder is a device in which extrusion is performed while applying mechanical stress by rotating 2 shafts. The mixing and dispersion properties for the additives is better via a biaxial extruder in terms of application of mechanical stress, which however is disadvantageous in terms of deterioration of the cellulose ester film. A uniaxial extruder in which mechanical stress is comparatively less is more advantageous with respect to deterioration of the cellulose ester film, but because extrusion is performed using the rotational force of one shaft, the mixing and dispersion properties for the additives deteriorate when compared to a biaxial extruder.
Methods for manufacturing a cellulose ester film in which a uniaxial extruder is used and in which mixing and dispersion properties for the additives are still excellent have been proposed (see for example Patent Documents 2 and 3). These methods results in pellets by heating and melting the cellulose resin ester and the additives at 200° C. or more in a biaxial extruder and then preparing a film by re-melting in a uniaxial extruder. In cases where these methods are used, deterioration of the cellulose ester resin accelerates due to the mechanical stress when the cellulose ester resin is subjected to double heating and then being pelletized in the biaxial extruder. When deterioration of the cellulose ester resin is excessive, the molecular weight as well as the mechanical strength decreases. This poses a problem not only in terms of product quality, but also in terms of recycling of the waste materials which are generated during production.    [Patent Document 1] Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) 2000-352620    [Patent Document 2] JP-A 9-241425    [Patent Document 3] JP-A 11-255959